South Africa's leading white liberal journalist, who was silenced by authorities last fall and placed under virtual house arrest, fled across the country's border in disguise last night.

Reached by telephone in Maseru, capital of neighboring Lesotho, the journalist, Donald Woods, declared jubilantly, "I now officially declare myself unbanned."

"Banning" is South Africa's system of isolating political dissidents from contact with the outside world. In Wood's case, the punishment meant he could not be quoted in South Africa, could not keep a diary or write, could meet with only one other person at a time and was extremely restricted in his movements.

For more than a decade, Woods, 43, had been a relentless critic of the South African governments apartheid policy of racial separation. As editor of the East London Daily Dispatch he wrote a syndicated weekly column read by half a million South Africans.

Woods was banned on Oct 19 in a major government crackdown on critics of apartheid that extended to six other people, 18 mostly black organizations and three publications.

After two and a half months of enforced oblivion, Woods said that he disguised himself yesterday by donning a mustache, removing his glasses and dying his normally sandy hair black. He hitchhiked from East London to the Transkel border and then to avoid border guards, swam across a rain-swollen river into Lesotho.

Woods' wife Wendy and their five children who left South Africa yesterday, ostensibly for a vacation in the Transkei, were expected to meet him tonight in Maseru where he is staying at the residence of the British ambassador.

Lesotho officials said Woods would be granted asylum in the country. "Our government has always welcomed people who have fled oppression in South Africa and I do not see that Mr. Woods is an exception," said Tom Tahabala, a senior spokesman.

Woods said by telephone that he planned to go to England and then to the United States where he would consider settling.

"The first thing we must do is sort things out," Woods observed.

He said he would probably look for a job in journalism and would write a book on the life of Steve Biko, the black activist leader whose death in detention earlier this year caused an international storm of criticism against South Africa and prompted the United States to support an arms embargo of the country by the United Nations. Biko and Woods were close friends.

Woods was considered to be one of the few whites who held the complete trust of the leaders of Biko's black consciousness movement, which advocates genuine political power-sharing between the country's races.

"I found it intolerable to remain [WORD ILLEGIBLE] Woods said today, [WORD ILLEGIBLE] I couldn't speak up within the country I had to get out."

My intention is to return to South Africa as soon as I can, as soon as there is change of government," he said. "Or a change of heart in the government."

Woods' flight surprised and saddened his close associates and friends, many of whom gathered at his East London home today to safeguard his personal effects. Some cried.

"This is the lousy part of the whole thing." Woods said of having to leave his friends behind.

He said only his wife and two oldest children had known of his escape plans since he made up his mind to flee a week ago.

About a month ago, one of Woods' children received a T-shirt in tha mail that had been dipped or sprayed with a skin poison. The child was hospitalized but recovered.

After an investigation, a detective whom Woods hired claimed that two government officials were responsible for the poisoning. Their names were not disclosed, however.

Briceland, managing director of the [WORD ILLEGIBLE] said he believed the incident was a major reason Woods had left.

"Donald is fearless," he said, "But when the attempt was made to poison the child that had more effect on him than anything I've seen in the 15 years I've known him. This depressed him enormously. He felt he could no longer trust those who were supposed to uphold the law."

Because he is banned, Woods can not be quoted in South Africa, and today the South African post office refuse permission to the Associated Press to transmit photographs of Woods or even of the family maid, who was left behind at the residence. Woods left her a large sum of money.

Woods had a column in several South African papers before he received his banning order. The column that would have appeared the day after the banning was a comparison of authoritarian government, lack of human rights and persecution of dissidents in South African and the Soviet Union.

Ironically, one of the main targets of the column was bannings and detentions.